The present invention relates to gain-guided type laser diodes, and more particularly to AlGaInP visible light laser diodes with oscillation wavelength not greater than 680 nm.
The gain-guided type laser diodes are attractive light sources for barcode readers in POS and FA systems. A conventional gain-guided type AlGaInP visible light laser diode is shown in FIG. 1 as reported in ELECTRONICS LETTERS 16th Jan. 1987 Vol. 23, No. 2, p.85.
Referring to FIG. 1, an n-GaAs buffer layer 2 is formed on an n-GaAs substrate 1. On top of the buffer layer 2 there is formed a double heterojunction structure consisting of an n-AlGaInP cladding layer 3, a GaInP active layer 4, a p-AlGaInP cladding layer 5, a p-GaInP etching stopper layer 6, a pair of n-GaAs current blocking layers 7 and a p-GaAs contact layer 8.
Here, the fabrication step of the conventional structure shown in FIG. 1 will be described. First, a six-layer structure from the n-GaAs buffer layer 2 to the n-GaAs current blocking layer 7 is grown sequentially by using a first vapor phase growth technique such as MOVPE (Metal-Organic Vapour Phase Epitaxy). Subsequently, a trench stripe 9 is formed in n-GaAs current blocking layer 7 to expose a stripe region of p-GaInP etching stopping layer 6. Over the entire surface of n-GaAs current blocking layer 7 including stripe 9 is formed by the second MOVPE growth a p-GaAs contact layer 8 over the entire surface of which in turn a p-side ohmic electrode 10 of Ti/Pt/An is deposited. Further an n-side ohmic electrode 11 of AuGeNi is deposited on the entire undersurface of substrate 1.
In this prior structure, the current is constructed by the p-GaAs contact layer 8 and the GaAs current blocking layer 7. The p-GaInP etching stopper layer 6 serves, for forming a stripe 9 in n-GaAs of current blocking layer 7 by chemical etching, to stop the etching to prevent from being over-etched, and has another purpose of reducing electrical resistance between the p-AlGaInP cladding layer 5 and the p-GaAs contact layer 8. In such a gain-guided type laser diode, injected carriers causes a refractive index difference in the active layer to guide a light along with the stripe direction.
Although the above stated gain-guided type laser diodes can be made easily compared with a buired hetero structure type laser diodes, so called kink is appeared in its current-light output characteristic when the output is increased. This is because the active layer of the gain-guide type laser diodes has no waveguide mechanism of refractive index difference in a direction parallel to pn junction plane. When the output is increased, a transversal mode of the output light becomes unstable and causes the kink.
The conventional gain-guided type laser diodes shown in FIG. 1 causes the kink in a low output light level. When the gain-guided type laser diode is used for a light source of a barcode reader, the position of the kink level is significant. The light output level must be kept below the kink level, otherwise the barcode reader will cause a miss reading. To this end, it is desired to increase the kink level as high as possible.
It is known that the kink level can be increased by narrowing the stripe width of the laser diode. But when the stripe width is decreased, a threshold current is drastically increased. In view of the threshold current, the stripe width should be selected about 7 .mu.m. Despite of such suitable stripe width, the kink level of the conventional gain-guided type laser diode is still low such as about 3 mW as shown in FIG. 3.